It is known in the ophthalmological field, that oxygen from the air must be made available to the eye in order to provide for the metabolic needs of the cornea. The placement of an oxygen impermeable contact lens over the cornea can seriously inhibit the transmission of oxygen to it and result in corneal trauma. This situation has been partially alleviated by the so-called pump design of corneal contact lenses which serves to replace the lachrymal fluid under the lens (containing carbon dioxide) with freshly oxygenated lachrymal fluid and thereby make oxygen available to the cornea. The oxygen deficiency problem has been further alleviated by limiting the length of time for which an impermeable lens can be continuously worn, ranging from about four hours to 16 hours of daytime wear depending on the individual, and no wear during sleeping hours. There is also a concomitant need for the carbon dioxide, which is produced by the corneal metabolic processes, to be carried away from the cornea.
It has been found that certain hydrophilic gel contact lenses of high water content have gas permeability orders of magnitude greater than conventional polymethylmethacrylate lenses and provide for the passage of the aforesaid gases through the lens in sufficient amounts to supply corneal metabolic needs. The term "gas permeability" as used herein refers broadly to air, oxygen and carbon dioxide permeability. It has been found, that the gas permeability increases with increasing water content of the hydrophilic polymer from which the contact lens is formed. The increased gas permeability makes possible increased wearing time with increased comfort and absence of most of the undesirable physiological symptoms produced by conventional lens wear.
In order to optimize the foregoing advantages of these hydrophilic lenses, materials with very high water content are desired. However, previously the strength of lenses with very high water contents, e.g., 60%-95% have been found to be low, the strength (resistance to tearing, puncturing, etc . . . ) decreasing progressively with increase in water content. Such lenses are readily damaged during handling, in general are not very durable, and have poorer visual acuity. In some instances such contact lenses can only be inserted and removed by a professional practitioner because of their fragility. Further, the high water content lenses of the prior art cannot withstand repeated heat disinfection or sterilization without deterioration or destruction of the lens. Polymers of the prior art which are of sufficiently high water content to give high gas permeabilities, e.g., about 60%-95% by weight of the combined weight of polymer plus water, and especially above about 70% water content, are very weak and are readily torn or otherwise physically damaged during handling. Such polymers are exemplified in British Pat. No. 1,391,438 and U.S. Pat. Nos. 3,639,524 and 3,943,045. The polymers are prepared from monomer compositions which contain a relatively high amount of cross-linking monomer, the latter being required to prevent the hydrophilic polymer from substantially dissolving in aqueous media. This excessive cross-linking frequently results in a weak polymer.
It has been desired to provide hydrophilic polymers and contact lenses made therefrom which are of high water content, high strength, good durability, high gas permeability and which can be repeatedly disinfected or sterilized by thermal means without deterioration or destruction or impairment of their optical properties.